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most part of this rock consists of thin, almost slaty laminas, with a curv- 
ed or contorted structure (see figure) resembling short undulations, gen- 
tly curved on the upper surface, but meeting in the depression or con- 
cave very abruptly. In some cases the surfaces of these curved laminae 
are covered with small knobs or protruberances, often arranged in quin- 
cunx order. These thin layers are separated by a thin seam of bitu- 
minous matter; they are very brittle, and when struck with a hammer 
give a metallic sound, like clinkstone. 
The name swinestone, or fetid limestone, is peculiarly inapplicable to 
this rock, as it has nowhere a fetid odor. The odor produced on per- 
cussion is that of bitumen, and the same is perceived on burning the 
stone for lime, when the fluid bitumen, of the consistence of tar, es- 
capes and is consumed, giving rise to the odor which has been termed 
that of burned horns or leather. 
In the eastern part of our district, this limestone has lost its geodife- 
rous character, and has greatly diminished in thickness; but it still re- 
tains its essential lithological character and bituminous odor. The term 
geodiferous, which distinguishes the western portions of this rock, is 
here inapplicable. 
This limestone is highly magnesian throughout its whole extent; still 
no portion of country can be more fertile than that along the range of 
this rock. The northern limit of this limestone from the Genesee to 
the Niagara river, is the edge of the limestone terrace. East of the Ge- 
nesee river its northern limit is in a line drawn from Rochester to Wol- 
cott, in Wayne county. Its width is from four to eight miles. 
SECTION VI. 
Gypseous Marls and Slates. 
Superimposed upon the rock just described, we find an extensive se- 
ries of slates, indurated marls, siliceous and argillaceous limestones, the 
whole formation marked by the predominance of argillaceous matter. 
This formation is peculiar and well defined, resting on the geodiferous 
limestone; it terminates with the mountain limestone, which is one of 
